1. Field of the Invention
The present invention relates to the equipment for the realization of semiconductor devices, and more particularly it concerns a vapour generator for chemical vapour deposition systems.
2. Description of the Prior Art
It is known that many electronic devices, and more particularly many optoelectronic devices used in the optical fiber transmission systems, such as laser, photodetectors, etc., are made up of structures of semiconductor materials comprising different monocrystalline layers with a varying composition and with thicknesses ranging from fractions of a micron to a few microns.
The individual layers are obtained by depositing the suitable compounds on a starting substrate; different methods can be used for the operation, among which the deposition method from vapour-phase reagents, called MOCVD (Metal-Organic Chemical Vapour Deposition), is rather widely used.
According to this method the possible sources of metals and of non-metals are some metalorganic compounds and pure elements, such as mercury.
During deposition, with the help of carrier gases such as, H.sub.2, Ar, vapours of these reagents are transported into the reactor where the starting substrate is contained, and kept at a suitable temperature. Here they react with one another and give origin to the required semiconductor material which deposits on the substrate. To obtain different layers the flow of reagents of a certain composition is replaced with the flow of reagents of a different composition, trying, at the same time, to keep all the other physical conditions unchanged, to avoid the formation of layers whose composition is not controlled.
The composition of the individual layers depends on the quantity of vapours reacting inside the reactor and therefore on the quantity of vapours which are extracted from the relevant sources and led to the reactor. Extraction of vapours is performed in special generators called bubblers if the source is a liquid, or sublimators if the source is solid. Extraction efficiency depends on different factors, the main of which are the carrier gas flow running the vapour generator in which the source is contained, the temperature and the pressure at which the source itself is kept.
One of the problems arising in transporting vapours from the generator to the reactor is their condensation on the internal surface of the ducts, specially if they are kept at room temperature. There are no drawbacks when the required quantity of vapours is very high, and the source temperature in the generator can therefore be kept at temperature values which are lower than room temperature. As a matter of fact, the mixture of the carrier gas saturated with source vapours goes out from the generator and is let into the ducts with a higher temperature, where it is heated and it becomes unsaturated enabling it to be led to the reactor without any danger of condensation.
When a greater quantity of vapours is required or reagent vapour pressure is very low, the temperature necessary to produce sufficient quantities of vapours must be higher than that of room temperature. In this case condensation can be avoided by heating the ducts at the output of the vapour generator so as to bring them, at least to the first dilution point, to a temperature which is higher than that of the container.
The dilution with inert gas of the saturated mixture of the reagent makes the mixture unsaturated and its condensation temperature becomes therefore lower. When using the generators available on the market, such as those manufactured by the Morton International Company (U.S. Pat. No. 4,506,815), the mixture is normally diluted at the output of the container after running a section of the system line, comprising certain components such as valves, pipes, fittings, etc.
This section of the line must be provided with a heating system assuring a constant and controlled temperature to avoid both condensation, and vapour decomposition at too high temperatures. Obviously a system like this involves a remarkable complication during the construction stage of the system.